1. Field of the Invention
The present invention relates to a spray dryer for drying instantly a process fluid containing solid material dissolved as an aqueous solution or dispersed in water as a slurry by spraying under pressure the process fluid from spray nozzles into a hot gas stream, and it specifically relates to a spray dryer capable of processing a large amount of the fluid in spite of the drying chamber of a lowered height.
2. Description of the Prior Art
A tower type spray dryer can characteristically produce dried powder instantly by spraying a process fluid from an upper portion of the drying chamber into a hot gas stream so as the fluid is pulverized and dried under increased surface area per unit weight. Especially, in the so-called parallel-flow type operation where an inlet of hot gas is disposed around an inlet of process fluid (fluid to be dried) so as the pulverized fluid is entrained by the hot gas stream, the operation is characterized by that most of the water contained evaporates at an initial stage of drying occurring in the neighborhood of the inlet of hot gas and no remarkable temperature rise occurs for the pulverized fluid even when the drying gas stream is so hot as around 200.degree. C. The drying tower must have a reasonable height as the drying should finish completely before the powder reaches bottom portion of the dryer. Nevertheless, due to the simple structure and easy operation, the drying apparatus (tower) is employed widely in food, pharmaceutical and chemical industries as the most simplified methods of processing a large amount of raw material sensible of heat or apt to be blackened.
For materials which are stable at high temperatures of above 400.degree. C. like detergents, a counter-flow type drying operation where a hot gas stream is countered with a pulverized process fluid in a drying chamber is employed. However, since the operation does not utilize the original merits of spray dryer, the counter-flow type drying operation is employed only in limited fields.
Conventionally, most of the parallel-flow type spray dryer has at the center of head portion (ceiling portion) of the cylindrical vertical drying chamber a single inlet of hot gas for blowing a hot gas for drying downward into the drying chamber. For a spray dryer processing a relatively small amount (evaporating load of not larger than 2000 kg/hr), a single rotary atomizer or a nozzle for spraying pressurized process fluid is disposed at the center of inlet of hot gas. In a larger capacity spray dryer, rotary atomizers are not employed usually but a plurality of nozzles for spraying pressurized process fluid are disposed toward a single large diameter inlet of hot gas.
When a rotary atomizer or a plurality of nozzles are employed for improving the drying efficiency of drying chamber, a rotating board for generating a spiral gas stream is disposed at the inlet of hot gas, however, the rotating board is not usually equipped for spraying a process fluid with a single nozzle. The pressurized process fluid sprayed into the hot gas stream from rotary atomizers or spraying nozzles is atomized and dried instantly in the drying chamber, and the exhaust gas is withdrawn from a lower portion of the drying chamber to enter a cyclone collector to separate and collect the dried fine particles in the exhaust gas. The collected fine particles are discharged outside of the system via a rotary valve or a double dumper disposed at a conical lower portion of the cyclone. After separating the fine particles with the cyclone, the exhaust gas is sucked via a exhaust gas pipe by an exhaust gas fan and discharged into the atmosphere.
Heretofore, rotary atomizers are employed for small scale cylindrical vertical spray dryers having a vapory water load of not larger than several tens kg/hr, and pressurized spraying nozzles are not employed. By the rotary atomizer, the process fluid supplied on the central portion of a disc rotating at an ultra high speed flows on the surface or through channels toward the periphery of disc and splashes away in fine fluid particles from the disc edge toward the radial direction (horizontal direction) by means of the centrifugal force. Since the drying hot gas is caused to rotate by the rotating disc, the flow down velocity of the particles in the axial direction of cylindrical chamber is small. Thus, the ratio of diameter to height of the cylindrical chamber including the conical bottom portion is around 1:2. Small spray dryers of this scale are basically installed indoor on the ground floor of housing.
In case of a pressurized spraying nozzle or a two kinds fluid spraying nozzle is employed, the process fluid is sprayed downward in the direction of chamber axis and at a speed of several tens m/sec from the spraying nozzles settled at the center of the inlet of hot gas on the ceiling portion of the drying chamber. The height of drying chamber is considered to be several times as high as that of spray dryers employing rotary atomizers because of the necessity for having the retention time in the drying chamber. In order to make the volume of drying chamber to be the same as that of employing rotary atomizers, it is necessary to halve the diameter of drying chamber, however, adhesion of wet particles on the wall of drying chamber prevents reducing the diameter of drying chamber too small to result in a lowered drying capacity per unit volume of drying chamber. Thus, rotary atomizers have being considered heretofore as far more suitable for small scale spray dryers. Without relating to drying capacities, spray dryers having rotary atomizers are required to enhance the drying efficiency within the drying chamber by disposing a rotating board at the inlet of hot gas to cause a rotation of the gas steam in the drying chamber. Though the rotating gas stream may be useful for enlarging effective regions of the drying chamber, the gas inevitably causes a trouble of depositing entrained fine particles on walls around the inlet of hot gas. The deposited particles encountered by the hot gas turn to char in a short period of time. Since the char grows gradually to layers of char and the grown layers dismember occasionally to result in inclusion of the char in the product, which constitutes fundamental deficiency in those spray dryers of difficultly operable for a long period of time.